Question:
How can I get a letsencrypt certificate installed on my new NextCloudPi installation, running on a Raspberry Pi in my home?

Brief Details:
My NextCloudPi instance is working well. I can access it from outside my home network via freedns using my subdomain name hazybluesky.mooo.com. However, of course I get the browser warnings about a self-signed certificate. I have tried installing letsencrypt certs both from the command line (see below) and using a script/wizard from the NextCloudPi admin panel. However, I consistently get the same error about http-01
challenge timeout. The error message suggests a firewall problem. However, my firewall does not block ports 80 or 443.